The present invention relates to a storage system and, more particularly, to a storage system that utilizes a plurality of removable storage media each subject to a predetermined maximum allowable update count.
Storage devices frequently used by computer systems include magnetic tapes and optical storage units in addition to magnetic disk drives. With each of these storage devices, media for data storage and a drive for writing and reading data to and from a medium are physically separated. A storage medium is loaded into the drive for a data write or read operation. These media are generally called removable media. One such medium attracting attention today is the DVD (Digital Video Disk).
Some media including the DVD-RAM and flash memory are each subject to a maximum allowable update count. The reason for this is that regions of a medium where data updates are repeated are prone to become unstable in data retention. If worst comes to worst, data will be lost through repeated updates. For example, the DVD-RAM may typically have its contents updated up to about 100,000 times; the flash memory, 105 times.
Japanese Patent Laid-open No. Hei 9-54726 discloses a storage apparatus enabling a flash memory subject to a maximum update count to be utilized more efficiently than before. The disclosed storage apparatus employs a flash memory in which data are erased in units of blocks each made of a plurality of sectors. Each block is assigned spare sectors. If data cannot be written to a sector, they are written to a spare sector of the same block. In that case, the logical addresses of the sectors are switched. If all spare sectors of a block have been exhausted, all data of that block are copied to an unused block, and the logical addresses of all sectors involved are switched.
One disadvantage of the conventional storage apparatus outlined above is that it has no provision for a case where all available blocks (and their spare sectors) have been used up within the flash memory, i.e., where all spare storage regions have been exhausted. In that case, the flash memory chip could conceivably be replaced by another chip. However, usually fixed to a board inside the storage apparatus, the flash memory chip is not easy to replace. On the other hand, if the storage apparatus utilizes removable media such as the DVD-RAM, the media are much easier to replace than the flash memory, except that they must be removed manually.
It is therefore an object of the present invention to provide a storage system utilizing removable media each subject to a maximum update count, wherein the media are replaced without intervention of an operator.
In carrying out the invention and according to one aspect thereof, there is provided a storage system comprising: a cabinet for accommodating a plurality of rewritable removable media; reading and writing means for performing read and write operations on the removable media; a transporter for transporting the removable media between the cabinet and the reading and writing means; and a controller for controlling the reading and writing means and the transporter. The controller manages an update count for each removable medium being used and, if the update count is found to exceed a predetermined threshold value for a first removable medium, copies contents of the first removable medium to a second removable medium so that after the copying, the second removable medium is used in place of the first removable medium.
In a preferred structure according to the invention, the controller may manage use status of spare regions in each removable medium instead of its update count. If, in response to a write request from a host computer, an attempt is made to update regions of a removable medium and fails, then the controller may update the use status of the spare regions of the removable medium in question while copying contents of the regions whose update failed to the spare regions. Thereafter the newly copied spare regions may be used in place of the failed regions. If an available size of the spare regions is found to be less than a predetermined threshold value for the first removable medium, the controller may copy contents of the first removable medium to the second removable medium.
According to another aspect of the invention, there is provided a storage system comprising: a cabinet for accommodating a plurality of rewritable removable media; at least two drives for performing read and write operations on the removable media; a transporter for transporting the removable media between the cabinet and the drives; and a controller for controlling the drives and the transporter. The controller holds update count management information for managing an update count for each of the removable medium being used. In response to a write request from a host computer, the controller causes the transporter to transport a first removable medium to which to write data from the cabinet to a first drive. Every time a data write operation is performed on the first removable medium, the controller updates update count management information by which to manage an update count for each of the removable media. When the update count for the first removable medium is found to have exceeded a predetermined threshold value, the controller causes the transporter to transport a spare second removable medium from the cabinet to a second drive. The controller copies data held on the first removable medium to the second removable medium before causing the controller to transport the second removable medium to the cabinet for storage therein in place of the first removable medium.
In another preferred structure according to the invention, each removable medium may be divided into a plurality of regions each subject to management of an update count for the medium in question. Alternatively, update count management information may be divided and assigned individually to the removable media being used, so that update count management information about a given removable medium may be stored on that medium.
In a further preferred structure according to the invention, the storage system may comprise an ejecting element for ejecting the removable media. The controller may cause the transporter to transport to the ejecting element the first removable medium whose contents have been copied to the second removable medium. In an even further preferred structure according to the invention, the storage system may comprise a loading element for loading removable media from outside. The controller may cause the transporter to transport a removable medium placed in the loading element to the cabinet for storage therein as a spare removable medium.
If it takes a long time to copy data to a spare medium, the copying process may be halted temporarily to accept read/write requests from the host computer. In such a case, two kinds of information need to be retained: information for identifying a copy source medium and a copy destination medium (e.g., copy source medium number and copy destination medium number), and information denoting the progress of the copying process up to the point of interruption (e.g., copy start record number).
According to a further aspect of the invention, there is provided a controlling method for use with a storage system having a plurality of rewritable removable media, comprising the steps of: managing an update count for each removable medium; if the update count is found to exceed a predetermined threshold value for a first removable medium, copying contents of the first removable medium to a second removable medium; and using the second removable medium in place of the first removable medium after the copying.
Preferably, not an update count of each removable medium being used but use status of its spare regions may be managed by the inventive controlling method. If an attempt to update regions of a removable medium fails, the use status of the spare regions of the removable medium in question may be updated while contents of the regions whose update failed are copied to the spare regions. Thereafter the newly copied spare regions may be used in place of the failed regions. If an available size of the spare regions is found to be less than a predetermined threshold value for the first removable medium, contents of the first removable medium may be copied to the second removable medium.
Preferably, the first removable medium whose contents have been copied to another removable medium is ejected out of the storage system. A removable medium may be loaded into the storage system from outside so that the loaded medium may be used as a spare second removable medium.
Other objects, features and advantages of the invention will become more apparent upon a reading of the following description and appended drawings.